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Abstract

Repetitive manual handling is a common cause of work-related back disorders, which include disc injuries, such as herniation, and low back pain. Disc herniation commonly occurs in younger individuals aged 25-55 years old as a result of repetitive stressing of the low back, which is associated with lifting. Despite its high resilience, intervertebral discs can be damaged during repetitive loading, causing cumulative damage and leading to injuries. Few in vitro studies have developed a mechanistic understanding of disc herniation using macro and microstructural analysis of disc tissue via imaging after repetitive loading. However, the initiation and propagation of disc herniation are still poorly understood since prior studies used non-physiological loading regimes that may impact the mechanism. In addition, no internal disc mechanics were measured during the applied repetitive loading to give an insight into disc tissue deformations and its association to disc injury. Therefore, the overall aim of this thesis was to determine the effects of simulated repetitive lifting and the resulting tissue failure on internal disc strains, and six degrees of freedom (6DOF) disc mechanics.

To achieve this aim, thirty cadaver lumbar functional spinal units (FSUs) were subjected to a sequence of 6DOF testing followed by simulated repetitive lifting (safe or unsafe) and repeated 6DOF testing. To measure mechanical properties before and after repetitive lifting, the FSUs underwent ±6DOF tests at 0.1 Hz for 5 cycles. Each specimen underwent an equivalent of one year of simulated repetitive lifting under safe (15 FSUs) and unsafe (15 FSUs) levels of compression, in combination with flexion, and right axial rotation for 20,000 cycles or until failure. Safe or unsafe lifting was applied as a compressive load to mimic holding a 20 kg weight either close to, or at arm’s length, from the body, respectively. Internal disc strains were measured using a wire-grid and radiosterometric analysis. After testing, failure and disc damage were assessed from comparing pre- and post-test magnetic resonance imaging (MRI) and macroscopically.

For the first time internal disc strains, specifically maximum shear strains (MSS), were measured during simulated repetitive safe lifting. Largest magnitude of percent shear strains was found in the anterior (76%), posterolateral (64%), and left lateral (60%) regions. In addition, MSS only increased by 8% from cycle 1 to cycle 20000, demonstrating the resilience of the disc. After simulated safe repetitive lifting, 73% of specimens were injured, having failure modes of an annular protrusion, endplate failure, or disc herniation. Largest magnitude of percent shear strains, annular protrusion and herniation were found in the posterolateral regions, which is consistent with clinical observations. The disc mechanics were also altered after simulated safe repetitive lifting. Decreases in stiffness were found in compression, flexion, and lateral shear with an increase in extension. For phase angle, increases were found in compression, left axial rotation, and posterior shear with a decrease in extension. However, there was no association between failure mode and mechanical properties. The changes in mechanical properties could be caused by a combination of deformation in anterior annulus due to the applied flexion, tissue damage, reduced disc height, and a migrating centre of rotation.

Internal disc strains and mechanics were then measured under simulated unsafe repetitive lifting. This loading regime led to 60% of the specimens failing via endplate failure with no disc herniation observed. The largest magnitude of percent MSS was found in the posterolateral (72%) and anterior (70%) regions, which was similar under safe lifting. The MSS of disc protrusion and endplate failure specimens was larger than the no injury specimens, suggesting the existence of a damage threshold of 50% strain. Above this threshold, the risk of tissue damage associated with disc injuries such as disc protrusion and endplate failure increased. Changes in disc mechanics were observed after simulated unsafe repetitive lifting in all 6DOF directions, however, they were no different than those found during safe lifting. The lack of differences between the two groups suggests that measuring the 6DOF mechanics alone was not sufficient to detect tissue level and regional disc damage.

Finally, the associations between internal disc strains, failure mode, and disc damage assessed via MRI and macroscopically for both lifting groups were examined. It was found that MSS increased with progression of disc injury towards herniation in the left lateral region under a combination of repetitive compression, flexion and right axial rotation. This finding provides evidence that the lateral region is more vulnerable than previously understood. Correlations between MSS and tissue damage further verified current clinical knowledge, where damage associated with disc protrusion and disc herniation was predominantly located in the posterior and posterolateral regions. This research further demonstrated that unsafe lifting places the disc at greatest risk of injury.

The disc mechanics measured within this thesis are vital in providing guidelines for finite element models to conduct studies in disc injury and repair, as well as optimising future disc replacement designs. Clinically, this research provides unique insights into the structure-function-injury relationships of disc tissue during repetitive lifting, and in future, may contribute to recommending important preventative lifting strategies in workplace guidelines to minimise the risk of herniation.